crack a window
When I press the defrost button in my car, it turns both the heat and AC on at the same time. I assume this is to remove water from the air with the AC, then warm up the air increasing its capacity to hold water with the heat.
Your assumption is correct Also your car automatically calculates the condensation point and keeps the temperature and air moisture low enough not to cause any fogging on your windshield. Source: I’ve been programming AC unit controls for cars for the last 10 years
in a lot of newer cars they can actually use the A/C to heat and cool the car, rather than using excess engine heat or an electric heater which is especially helpful when it’s cold and you’ve only just started the engine!
Fun fact (well, I suspect you know this already, but some didn’t, I had discussion with someone arguing with me that this is not true):
AC and a heat pump are basically the same thing, AC is just pumping the heat outside instead of inside.
Also when you say modern cars I am assuming you might be talking about EVs. Using a heat pump requires less energy than using resistive heating so it could be energy saving measure.
ICE cars on the other hand can get the heat from engine “for free” (not really, but since the heat will be produced anyway whether you use it or not, so why not use it?) so a heat pump might make a bit less sense in them.
Air-conditioners are in a way dehumidifiers, so that makes sense
Every time. And I’m halfway through life. I could do a web search, but who ever thinks of that except when this happens? Will I do a web search now? No. I have other stuff to think about. And so it goes.
Atleast you have a car
The best situation is when you don’t need a car to simply exist. That should be the norm, not the exception.
Always use heat. Anytime you have a fogged up window heat will cause that moisture to evaporate. Cold will always make the window cooler which causes more condensation.
Air conditioning will remove moisture from the air while cooling the cab, so the condensation can only form on the outside, where you have wind and windshield wipers to wipe off the condensation.
67o
Bro turns his car into a sauna
The fog is moisture condensing on the cold windshield surface. Use heat directed at the windshield with AC on. The AC dries out the moisture and the heater warms up the surface preventing additional condensation.
If it’s already cold, you don’t need to turn the AC on. The air will be dry once it gets warmer.
Defrost setting uses the AC unit. Helps pull moisture out of the air
Air conditioners pull moisture out of the air by making it colder, reducing the amount of moisture the air can hold. Its not doing anything if the air is already colder than the air conditioner can go. The air conditioner will only help if it’s 55F/13C or more, where it will only fog up if you live somewhere very humid.
I know Google and the AI say otherwise. I am an HVAC engineer, and Google and the AI are wrong.
I don’t have a side in this but both my ‘93 Ford Escort and my ‘01 Ford Crown Victoria would turn on the A/C compressor if you hit the Defrost button for the windshield.
Edit: Just searched it and I think you two are in a semantic misunderstanding.
The moist air freezes to the coils of the ac unit, which is than heated by heated air and drips out of the vehicle through a condensate drain.
How do you not understand that basics of an air conditioner? It doesn’t just cool, it conditions the air.
That doesn’t even make any sense. Is it cold air depositing frost onto the coil, or hot air melting the frost? It can’t be both. You know that frost building up on coils is a huge problem that breaks air conditioners, right? Frost clogs the air stream, and creates a feedback loop where the coil gets way colder than it’s designed to because air can’t pass through.
I could have been more specific: the air conditioner can be useful in rainy or foggy warm conditions where the air conditioner can still cool the air further, and a car air conditioner goes colder than a home air conditioner does, around 38-45F (3-7C). It can also help if something in the car is adding moisture to the air, like if the occupants are sweating. However, when it’s just a cold day fogging the windows, the air conditioner does not help. It does not have a mechanism to dehumidify the air other than to cool it. Turn the temperature up to warm the interior surface of the windows.
Uhhh… why do you think they have defrost cycles?
If you turn off the ac for a short period of time, you run heat over the coils to remove all the accumulated frost, snd switch over again. This just used air instead of heating the coils with a reverse loop. It constantly cycles this loop to accumulate frost and remove it. It can be both, the coils are colder than the air, it accurate condensate, just like your glass. Why does your glass collect moisture if you claim this scenario is impossible….?
You say others are just using Google and ai, but clearly you’re the one using those tools and have no experience. Or did you just start school and it’s your second day?
Coil freeze over like you’re talking about is from an improperly sized unit, the defrost cycle can’t work properly.
But all acs collect moisture and have condensate lines, even home ones
Car defrost cycles are explicitly for the windows, not the coils. You still haven’t explained the magic mechanism by which an air conditioner dehumidifies air that’s colder than the coils, just pointed to the existence of condensate like that makes your point somehow. I already covered that the air conditioner can dehumidify air cycling through the car if humidity is building up from something inside it.
The coils are not designed to freeze. Ever. Having to manually defrost the coils by turning off the A/C (you have to do it manually because no system is designed to work this way, because that would be a stupid design) and run warm air through it means your system is malfunctioning.
Take the last word, if you want. Arguing on the internet is pointless.
Wouldn’t it be more efficient to pass the cold dry air back over the hot side heat exchanger to increase its moisture caring capacity before blowing it at the glass?
You know like a dehumidifier?
Probably, but the hot side of the car’s AC is outside of the car typically in front of the radiator.
I want to be able to blow hot air at my ice cold windshield in the -20°C Canadian winters, but American car designers are all like “oh you want to use the windshield vent? you must want the AC turned on!”
This isn’t just an American car thing, non American brands do this as well. That’s because it’s a good idea. The heater core has way more capacity than the AC, so the air coming out of the vent will still be warm. But running the AC is still a good thing, because it gets the moisture out of the air.
Without running the AC the humidity in the car shoots up and takes a while to clear, with the AC on the humidity stays controlled. This helps clear the windscreen sooner and helps with comfort in the car. It also prevents mold, which if you live in a cold but humid place, can be very annoying. It is very normal where I live to have the AC on in fall and winter, because otherwise the humidity inside the car is very uncomfortable and can easily cause mold all over the interior.
If it’s a cold winter day it’s dry outside. Just open your cabin circulator to bring in outside air, and you don’t need to use the extra gas to run the AC.
If you’re in an equatorial area, foggy windows will be because of unbelievably high humidity in hot to boiling weather, which WILL require AC because you can’t just suck in more wet air and make it hotter and expect that to work.
Is -20°C regular winter temps for Canadians? It’s colder than that here in Minnesota
Where most Canadians live it doesn’t get particularly cold. Toronto is south of Minnesota. There are places that get really cold, but not many people live in most of them.
This is wrong based on the assumption that it works the same in all areas of the world, but I’m not sure if this is -10 votes wrong.
Especially considering some of the other posts I’ve seen on here and how they get voted on
This can be correct in northern winters, with very high humidity this will only fog the windshield much worse.
You cant have high humidity when its cold, doesnt depend on the location.
100% humidity at 0°C is ~20% humidity at 25°, which is pretty dry.
I guess cold is a relative term. I’m from Wisconsin- we can have 100% humidity at 65 degrees sooooo
65°F so 18°C, and thats what you call cold? :D Then yeah, heater without AC will not clear your windshield up.
I’ve tried that. It only made things worse.
Clean your window.
Yup. The inside of the windshield gradually gets cloudy from all the plastics inside the vehicle slowly off-gassing. Soapy water, a squegee, and a microfiber do wonders.
Huh, that makes sense. I was wondering what caused that buildup slowly over time.
Temp isn’t as important as ac
If your windows gets foggy you need to turn up the heat as that lowers the relative humidity and direct the air towards the windshield, also check below the floor mats for water. If your car is a swimming pool your windows will be foggy forever.
Counter intuitively, you want to turn the a/c on as that pulls the moisture out of the car.
During winter when a vehicle is defrosting, it runs a combination of ac and heat to defrost windows and condition the air to limit moisture condensing on the interior.
If your car has that, then yes.
Remember to pull your sun visors down. My AC doesn’t stay charged so I have to rely on heat alone to defog windows. The visors trap the warm air.
Whichever of the AC/Heat is on, just switch it to the other one. Or turn it off. Or open the windows.
People in comments assume the heat or air conditioning even work.
Air conditioning also rapidly reduces humidity, so that tends to be the ideal option.
But if neither heat nor air conditioner work, well tough it out and roll the windows down…
If neither the heat or A/C work, you shouldn’t be driving that POS on the road.
Just because the ac/heat don’t work it’s automatically a POS that’s not road worthy?
If your car windows are totally fogged up, you cannot clearly see what is in front of your car and therefore, your vehicle is not safe to operate.
Depends on the part of the world and season. It wouldn’t be roadworthy to drive in freezing temps without heat on the windshield
Yes.
Our heater does work, just no blower motor, so if we need heat, just open the vents and drive for airflow.
Our air conditioner unfortunately does not work, but we have manual windows that roll down by hand for airflow.
And today I just happened to install a fan on the dashboard.
Yes it’s a piece of shit truck, but ignoring the ‘luxury’ features that cost more than the truck itself to fix, it’s pretty darn reliable.
See my previous posts regarding Kermit the 96 Sonoma for reference. More reliable than new model Ford trucks that brick themselves because rainwater got into the taillight assembly…
Your blower motor probably does work, but the resistor assembly hooked up to the knob that controls its speed is most likely blown. This was the case with my truck when I got it and is rather common. Thirty four bucks will solve that problem for you, and it just plugs right in.
The entire right side of the dashboard is missing, all the way down to the firewall, no thanks to the previous but now deceased meth tweaker owner.
There’s no fixing everything on this truck, short of dumping enough money into buying 4 more scrap trucks for spare parts…
That would not be street legal here. Must have a functional defroster if the vehicle has a windshield.
“Here” is an important word! I assume you are Canada based. I live where we rarely or ever get freezing temperatures, and only decently started having warm/hot summers. Why would defrosting or ac be mandatory in such climate?
It never rains there?
We have wiper blades?
On the inside of the windshield from condensation?
Well I take it we’re not in your state or country. Mississippi here, we no longer have vehicle inspections. And this is respected even in other states that do require vehicle inspections. They’re like yeah fuckit, half the windshield is cracked, but it’s from Mississippi, so nothing Louisiana can do.
Down here, we’re half-professional rednecks that try our best to keep our own vehicles running. As long as it starts, runs, stops, shifts gears, steers, and has all the critical lights working, it’s fine.
Hood, trunk, windshield, front grill, rear bumper, doors, heat, air conditioning, etc luxuries are all optional here. As long as it runs, stops, and steers is basically what all Mississippi cares about, plus lights at night. And other states have to observe our very lax vehicle requirements too.
I’m still working on some of the finer things, like it needs a horn, but generally speaking the truck runs pretty damn good to be 30 years old.
Uhhh the defrost button? Arr people that dumb?
Humid and warm: use heat
Cold and dry: turn on a/c
I think you got the right idea, just backwards…
Assuming your heater, air conditioner, and blower motor all work correctly, you should use your heater defroster in cold weather, and your air conditioner defogger in warm weather.
Note the difference in environmental control terms…
- DEFROSTER > Used for cold climate, heater, to melt the frost off your windshield.
- DEFOGGER > Used for warm humid climate, air conditioner, to dehumidify the air.
Failing either heater or air conditioner, welp, just do what we do and roll the windows down.
Both of those settings turn on the A/C to remove moisture.
Yeah… no… that’s only the newer generation of vehicles, the enshittified ones that assume everything for you.
Back in the 90’s and 2000’s, the engineers had common sense and if the temperature dial was set anywhere above 50%, the air conditioner would never turn on, as it’s a waste of energy at that point.
Uhhh my 96 civic had an air conditioner button, and it also turned on when you turned the defroster on.
You’re making shit up, it’s a simple interconnection, and the physics of air conditioning removing moisture has been known for a long time.
During winter snow constantly gets inside of a vehicle, so without an interconnected ac with defrost, the interior would always fog up. It just may not light up the button, but defrost turns the ac on.
Does absolutely nobody understand the difference between FOG vs FROST?
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The deFROSTer is meant to gently melt snow and ice FROST off your windshield using the heater. Kinda fucking counterintuitive to blow freezing air at a frozen windshield, the FROST will never thaw that way.
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The deFOGger is meant to dry the humidity off your windshield on a hot humid day, that uses the air conditioner on hot humid days.
Two different systems, for two different climates. deFOGger vs deFROSTer are two completely different modes of the climate controls.
Please Google “car defog” and see that there’s no results for a fucking button you muppet.
Thank you for the advice you leaking Tide Pod, there’s literally a fucking button…
“To effectively use the defrost and defog controls in a 1996 GMC Sonoma, press the Defog/Defrost button, turn off the Recirculation button, set the fan to the highest setting, and adjust the temperature to high. This will help clear the windshield quickly.”
Anyways, you drive and troubleshoot your vehicle, and we’ll drive and troubleshhot ours. They’re not all designed the same you muppet.
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My 78 Chrysler was designed to use the AC in defrost mode even in a winter blizzard and was much more effective and drying the windshield because of it. Most heating/cooling system in cars were combined much earlier than that to take advantage of dehumidifying the air. It also has the benefit to run the compressor to keep lubrication and stop seals from drying out in the AC system that would otherwise not be used to months at a time.
There are some vehicles with separate heating and AC systems, (mainly cheap or aftermarket units) but the vast majority run together. (At least in North American cars.)
deFROST != deFOG
- deFROSTer is meant to melt frost, ice and snow off the windshield, uses heater.
- deFOGger is meant to dry fog off your windshield on hot humid days, uses air conditioner.
I’m starting to gather that most people don’t have any clue how to properly use their environmental controls.
I really don’t know what to tell you there, I’m going by the vehicle manufacturer’s labels. The “defrost” mode on your climate controls (typically all the way to the right) that blows air towards your windshield will often use the ac compressor even if you have the heat set to high, because ac drys the air… it doesn’t have to be cold, it can be any temperature. Blowing wet warm air at the windshield isn’t very effective compared to dry warm air. That is especially true in our Canadian winters.
You can try it with virtually any car made from the 1970’s to today (and some in the 1950’s and 60’s) that was equipped with AC. Turn your defrost on, crank your heat, and you will hear/notice the slip clutch on the end of the AC compressor under the hood will kick in, turning on the ac unit. Some cars do come with a dedicated separate defrost button, (as opposed to the defogger button which only operates the heat lines in your back window) they also will use the AC to dry the air.
Defoggers are typically the heat lines you see in back windows, although some old cars (like my old 69 Newport) had blowers for the back window as well, and even a second heater core to feed it.
Long story short, AC is for drying air, not just cooling it.
Long story short, AC is for drying air, not just cooling it.
No shit, that’s my entire point.
Not everybody has fully working AI based environmental controls, some of us from Pepperidge Farms remembers when the controls were separate and not exactly automated the way lazy fucks expect these days…
My 2015 vehicle does it that way (a/c on for both), so it’s not exactly a recent change in design philosophy…
I said 90’s through 2000’s, I said nothing about 2010’s or beyond.
I do not have much of any experience in working on anything past 2005.
Anything past 2014 is what I call new generation enshittified, as that’s around the year they basically started tracking everyone, refusing to offer printed service manuals (Hayne’s/Chilton), stop offering parts and requiring ‘owners’ to take the vehicle back to the dealership for repair.
I’m all about the right to repair. Your 2015 is a new generation enshittified thing, sorry.
So why are you discounting what people who do use modern cars are saying as incorrect if you don’t know much about it? I’d wager most people probably aren’t driving 25-35 year old vehicles anyway so at best you’re just stating outdated and irrelevant knowledge.
Knowledge is never irrelevant if you’re still driving around a 30+ year vehicle. Knowledge is relevant across all years, here’s reference from 1960-2025 year range vehicles. I’m not fucking retarded…
What I do know, first hand experience, is if the air conditioner goes out before the heater core heats up, well you got about 15 seconds to roll the windows down for airflow to keep the windshield from fogging up.
In my old '90s car, heat only just made the fog worse. You had to have AC on too. Though it did trust you to know that and turn it on yourself.
And this is the proper way.
My 04 turns on the AC with the defrost, so did the 01 I used to drive, both base-level economy boxes so actually wait what the hell have you been driving?
Do people understand the difference between FROST vs FOG?
- Frost happens in the cold, best mitigated with gentle heat.
- Fog happens in the humid heat, best mitigated with air condition cool dry air.
FROST != FOG
